Method of ignition and corresponding ignition unit

ABSTRACT

The present invention provides an ignition method for an internal combustion engine, an injection being alternatively performed in at least one first operating mode or in a second operating mode, and the ignition coil being charged as a function of the current operating mode. A control-pulse curve characteristic of the current operating mode is provided, and the charging of the ignition coil is performed by a control logic element in response to the control-pulse curve, using corresponding, different time characteristics of the primary current. The present invention also provides a corresponding ignition device for an internal combustion engine.

FIELD OF THE INVENTION

The present invention relates to an ignition method for an internalcombustion engine, an injection being alternatively performed in atleast one first operating mode or in a second operating mode, and theignition coil being charged as a function of the current operating mode;and the present invention relates to a corresponding ignition device.

Although applicable to any fuels and engines of any vehicles, thepresent invention and the problem on which it is based are explainedwith reference to a direct gasoline-injection system of an engine of apassenger car.

BACKGROUND INFORMATION

FIG. 4 illustrates the dependence of torque M on engine speed N fordifferent operating modes of an internal combustion engine.

During so-called homogeneous, normal operation H1 of the directgasoline-injection system, the entire combustion chamber ishomogeneously filled with a stoichiometric air-fuel mixture (lambdavalue λ=1), which is ignited by the ignition sparks at the ignitionfiring point. In this case, there may be no ignition problems at allwhen the mixture has a high energy density.

However, homogeneous operation may also be realized in a lean mannerand/or with exhaust-gas recirculation (EGR) as homogeneous operation H2.In this case, a high level of flow may be required in order to achievesufficiently rapid burning in the case of low energy densities of themixture in the combustion chamber. This may deflect the spark plasma,until it breaks away and reignition occurs.

In this manner, the spark energy during coil ignition may be distributedwith typical spark durations of approximately 1 ms under theseconditions, to numerous, subsequent sparks, which each reach new mixtureregions.

But since the leanest operation or so-called high-EGR operation may onlybe attained when the entire energy of the ignition coil is introducedinto a single flame core, all of the energy stored in the ignition coilmay be required therefore to be supplied in such a short time that thespark still does not break away within this span of time (such as, forexample, approximately 0.3-0.6 ms.).

This may yield a demand for as high an energy as possible and a veryshort spark duration (approximately 0.3-0.6 ms) for this H2 operation,which may result in a high, required initial current of 150-200 mA.

In order to make use of the fuel-consumption features with internalcombustion engines having direct gasoline injection, so-called chargestratification may be implemented in the combustion chamber in certainoperating ranges, which is referred to below as stratified-chargeoperation S.

During stratified-charge operation S, only a small, locally ignitablestoichiometric cloud is introduced into the combustion chamber, whereasthe remaining contents of the combustion chamber may not be ignited. Afeature of this stratified-charge operation S may include that thelean-combustion operation of the engine is extended, and fuel maytherefore be saved in the end. Therefore, it may be desirable toconfigure the operating range of stratified-charge operation S to be aslarge as possible, and in particular, to therefore expand it to loadsand engine speeds that are as high as possible.

During stratified-charge operation S, marked local and/or temporallambda fluctuations may be present at the location of the ignitionspark, when the average energy density in the mixture cloud is high. Inorder to achieve reliable ignition in this case, the spark should burnfor a long time (such as, for example, approximately 5-10° KW (KW=crankangle)), so that within this time, the formation of the flame core maybe started when a flammable mixture region is seized by the sparkplasma.

In this context, depending on the flow of the mixture at the spark plug,only a continuously decreasing portion of the electrical energyintroduced from the ignition coil may be available for forming the flamecore as the spark duration increases. Thus, the conventional proposalmay generate a pulse train, i.e. to repeatedly charge and discharge theignition coil, within the above-mentioned KW interval.

Therefore, an individual ignition spark that burns as along as possiblewith an initial current of, for example, approximately 50-80 mA and asecondary energy of, for example, approximately 80-100 mJ, or anadjustable-length pulse train with an initial current of, for example,approximately 100 mA from a coil having, for example, approximately 30mJ of secondary energy, may be suitable for this stratified operatingmode.

Since the demands for stratified S and homogeneous H1 and H2 operatingranges may therefore be markedly different, a conventional systemconfiguration having individual sparks may create a conflict of aims,which may have previously only been approached as a compromise. Anignition coil may either be configured for a long spark duration (highsecondary inductance, i.e. high number of secondary windings per unitlength) with a moderate initial current, or for a short spark duration(low secondary inductance, i.e. low number of secondary windings perunit length). Therefore, a decision for a discrete configuration as acompromise may be essential.

SUMMARY OF THE INVENTION

In contrast to the conventional configuration approaches, an exemplaryignition method and/or exemplary ignition device of the presentinvention may provide that a functionality adapted to the problem ofdirect gasoline-injection engines may allow optimum ignition instratified operation, as well as in homogeneous lean-combustionoperation and/or with EGR, and in cold starting or other critical engineconditions.

The operating mode may be controlled as required. Only the amount ofenergy required for ignition may be introduced. This may preventspark-plug wear.

A smaller space for the coil due to a smaller number of turns per unitlength on the secondary side, or a larger iron cross section, may beprovided in the same space. Therefore, a cost advantage may be attainedby dispensing with the magnets for pre-magnetizing the iron circuit.

The type of ignition suitable for the specific operating mode may beprovided by control-pulse coding. For example, a pulse-train ignitionsuitable for stratified operation may be combined with the option ofloading the ignition coil with a markedly higher amount of energy duringhomogeneous operation by increasing the primary current, so that itstill discharges as a single spark within the desired spark duration ofapproximately 0.3-0.6 ms.

According to a further exemplary refinement, the first operating modemay be a homogeneous, normal operation, which may be divided up into thesubmodes of stoichiometric normal operation and sub-stoichiometricnormal operation, and the second operating mode may be an inhomogeneousstratified-charge operation.

According to a further exemplary refinement, the charging of theignition coil during inhomogeneous, stratified-charge operation may beperformed in the form of pulse-train ignition with a predeterminedprimary current, and the charging of the ignition coil duringhomogeneous operation may be performed in the form of a single-pulseignition with an increase in the primary current.

According to a further exemplary refinement, the control-pulse curvescharacteristic of the current operating mode may have different pulsetimes and/or numbers of pulses. Thus, virtually all operating states maybe coded, using a simple arrangement.

According to a further exemplary refinement, the iron circuit of theignition coil may be controlled up to the start of saturation, in anoperating mode that requires a high initial spark current. Thus, moreenergy may be stored and the rate of increase of the voltage may beincreased because of the lower, secondary inductance at the beginning.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a representation of the curve of spark current i_(F) versustime t according to a first exemplary embodiment of the presentinvention. FIG. 2 shows a representation of the curve of spark currenti_(F) versus time t according to a second exemplary embodiment of thepresent invention.

FIG. 3 shows a schematic representation of a control device forrealizing the first and second exemplary embodiments.

FIG. 4 shows the dependence of torque M on engine speed N for differentoperating modes of an internal combustion engine.

DETAILED DESCRIPTION

FIG. 1 is a representation of the curve of spark current i_(F) versustime t according to a first exemplary embodiment of the presentinvention.

In FIG. 1, curve a) represents the spark-current characteristic in theform of the discharge of the ignition coil (secondary energyapproximately 30 mJ, primary interrupting current approximately 10 A),without the pulse-train characteristic. The initial, secondary-sidespark current is approximately 110 mA with a spark duration ofapproximately 0.35 ms and a spark voltage of 1500 V.

Curve b) shows this ignition coil during the generation of a pulse trainhaving four pulses, in which, in each case, the primary-sidere-energization of the ignition coil occurs when the spark current hasdecreased to approximately 50 mA. A battery voltage of 42 V is assumedin order to realize the short recharging time.

In general, it should be mentioned that, in the case of a batteryvoltage of 14 V customary in conventional methods heretofore, the shortrecharging time may be achieved by increasing the primary current from10 A to 30 A.

Curve c) shows the spark-current characteristic for homogeneousoperation H1 or H2, namely when the coil is charged to approximately twotimes the energy, 60 mJ, by increasing the primary-side interruptingcurrent (from approximately 10 A to 15 A).

This yields a spark duration of approximately 0.5 ms, given an initialcurrent that is increased to approximately 160 mA.

This first exemplary embodiment assumes that the coil is in the linearrange of the magnetizability.

FIG. 2 is a representation of the curve of spark current i_(F) versustime t according to a second exemplary embodiment of the presentinvention.

In this second exemplary embodiment according to FIG. 2, it is assumedthat, due to the limited space (bar coil), a linear increase in themagnetizability may no longer be achieved, but rather the nonlinearityof the magnetization is intentionally incorporated.

Curve a) represents the spark-current characteristic as the discharge ofthe ignition coil (bar coil, secondary energy approximately 30 mJ,primary interrupting current approximately 10 A), without thepulse-train characteristic. As in the first example mentioned above, theinitial, secondary-side spark current is approximately 110 mA with aspark duration of approximately 0.35 ms.

As in the first example mentioned above, curve b) shows this ignitioncoil during the generation of a pulse train having four pulses, inwhich, in each case, the primary-side re-energization of the ignitioncoil occurs when the spark current has decreased to approximately 50 mA.In this case, a battery voltage of 42 V is likewise assumed in order torealize the short recharging time.

Curve c) shows the spark-current characteristic for homogeneousoperation, namely when the coil is charged to approximately two timesthe energy, 60 mJ, by increasing the primary-side interrupting current(from approximately 10 A to 20 A). This yields an increased initialspark current of 200 mA, which decreases in a nonlinear manner, i.e.more steeply at the beginning, since a lower inductance is initiallypresent on account of the saturation property. A sufficiently shortspark duration of approximately 0.5 ms may also be obtained in thiscase.

This configuration may have two features. When space is limited (barcoil), more energy may be stored when the iron circuit is activated upto the start of saturation. The rate of increase of the voltageincreases because of the lower, secondary inductance at the beginning.The increased rate of voltage increase may have a positive effect in thecase of spark-plug shunting, i.e. carbon-fouled spark plugs (coldstarting).

FIG. 3 shows a schematic representation of a control device forrealizing the first and second, specific exemplary embodiments.

In particular, MS designates an engine control unit, L a control logicelement, and ES an output stage, which includes a power transistor LT, aspark plug ZK, and an ignition coil ZS as fundamental components. It isassumed that the electronics which generate a pulse train, i.e. controllogic element L and output stage ES, are arranged on/in ignition coilZS.

A control pulse SI, which has a code from which control logic element Lmay locally recognize if a low-energy pulse train, a high-energy pulsetrain, a single, low-energy pulse, or a single, high-energy pulse isdesired, is supplied by engine control unit MS as a function of thecurrent injection mode.

FIG. 3 shows examples of suitable codes:

a) a single, short control pulse SI (approximately 10-100 μs): single 30mJ spark during homogeneous operation with λ=1;

b) two short control pulses SI (each approximately 10-100 μs): single 60mJ spark during homogeneous, lean-combustion operation, optionally withEGR;

c) a long control pulse SI (approximately 1-5 ms): pulse train base, 30mJ, during stratified-charge operation;

d) a long control pulse SI (ca. 1-5 ms) after a short control pulse SI(approximately 10-100 μs): 60 mJ pulse train base during cold startingand/or maneuvering, or under other particularly critical engineconditions.

Although the present invention is described above on the basis ofexemplary embodiments, it is not limited to them, but may be modified ina number of ways.

In particular, the present invention is not limited to the illustratedpulse shapes, energies, spark durations, and the like, but may begeneralized as needed. Further injection modes or different injectionmodes may also be provided.

What is claimed is:
 1. An ignition method for an internal combustionengine, comprising: performing an injection alternatively one of in atleast one first operating mode and in a second operating mode; providinga control-pulse curve that is characteristic of a current operatingmode; loading an ignition coil with energy as a function of a primarycurrent by a control logic element in response to the control-pulsecurve; and using corresponding, different time characteristics of theprimary current to produce ignition sparks, released by the ignitioncoil at a spark plug, differently for the at least one first operatingmode and the second operating mode; wherein: the at least one firstoperating mode is a homogeneous, normal operation that is subdividedinto submodes of a stoichiometric, normal operation and asub-stoichiometric, normal operation and the second operating mode is aninhomogeneous, stratified-charge operation, the loading of the ignitioncoil during the inhomogeneous, stratified-charge operation is performedas a pulse-train ignition, using the primary current, and the loading ofthe ignition coil during the homogeneous, normal operation is performedas a single-pulse ignition with an increase in the primary current. 2.The ignition method according to claim 1, wherein the control-pulsecurve characteristic of the current operating mode has at least one ofdifferent pulse times and different numbers of pulses.
 3. The ignitionmethod according to claim 1, further comprising: controlling theignition coil in an operating mode in which ignition sparks having ahigh initial spark current are required so that an iron circuit of thespark plug having a linear range of magnetizability is controlled up toa start of saturation of a magnetization.
 4. An ignition device,comprising: an ignition output stage; a control logic element connectedas an input to the ignition output stage; and an engine control unit forgenerating a control-pulse curve that is characteristic of a currentoperating mode; wherein; in response to the control-pulse curve, thecontrol logic element is configured to adjust the ignition output stageto a corresponding time characteristic of a primary current; at leastone first operating mode is a homogeneous, normal operation that issubdivided into submodes of a stoichiometric, normal operation and asub-stoichiometric, normal operation and a second operating mode is aninhomogeneous, stratified-charge operation, the loading of the ignitioncoil during the inhomogeneous, stratified-charge operation is performedas a pulse-train ignition, using the primary current, and the loading ofthe ignition coil during the homogeneous, normal operation is performedas a single-pulse ignition with an increase in the primary current. 5.The ignition device of claim 4, wherein the control-pulse curvecharacteristics of the current operating mode has at least one ofdifferent pulse times and different numbers of pulses.
 6. The ignitiondevice of claim 4, wherein the ignition coil is controlled in anoperating mode in which ignition sparks having a high initial sparkcurrent are required so that an iron circuit of the spark plug having alinear range of magnetizability is controlled up to a start of amagnetization.